Method for regulating a short-term power increase of a steam turbine

ABSTRACT

A method for regulating a short-term power increase of a steam turbine with an upstream waste-heat steam generator is provided. The steam turbine has a number of economizer, evaporator and super heater heating surfaces forming a flow path for a flow medium. The flow medium is tapped off from the flow path in a pressure stage and is injected into the flow path on the flow-medium side between two super heater heating surfaces of the respective pressure stage. Amount of flow medium injected is regulated with a characteristic value which is a discrepancy between the outlet temperature of the final super heater heating surface and a predetermined temperature nominal value. The temperature nominal value is reduced and the characteristic value is temporarily increased more than in proportion to the discrepancy for a time period of the reduction for achieving a short-term power increase of the steam turbine.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2011/065221 filed Sep. 02, 2011 and claims the benefitthereof. The International Application claims the benefits of Germanapplication No. 10 2010 040 623.6 filed Sep. 13, 2010, both of theapplications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a method for regulating a short-term powerincrease of a steam turbine with an upstream waste heat generator havinga number of economizer, evaporator and superheater heating surfaceswhich form a flow path and through which a flow medium flows, in whichflow medium is tapped off from the flow path in a pressure stage and isinjected into the flow path on the flow-medium side between twosuperheater heating surfaces of the respective pressure stage, with afirst characteristic value which is characteristic of the discrepancybetween the outlet temperature of the final superheater heating surfaceon the flow medium side of the respective pressure stage and apredetermined temperature nominal value being used as a regulationvariable for the amount of flow medium injected.

BACKGROUND OF THE INVENTION

A waste heat steam generator is a heat exchanger which recovers heatfrom a hot gas flow. Waste heat steam generators are frequently used ingas and steam turbine plants (combined cycle power plants) which areused predominantly for electricity generation. In such cases a moderncombined cycle power plant usually comprises one to four gas turbinesand at least one steam turbine, wherein either each turbine drives agenerator in each case (multi-shaft system) or a gas turbine drives thesingle generator with the steam turbine on a common shaft (single shaftsystem). The hot exhaust gases of the gas turbine are in such cases usedin the waste heat steam generator for generation of water steam. Thesteam is subsequently fed to the steam turbine, usually around twothirds of the electrical power is allocated to the gas turbine and onethird to the steam process.

Similarly to the various pressure stages of a steam turbine, the wasteheat steam generator also comprises a plurality of pressure stages withdifferent thermal states of the water-steam mixture contained therein ineach case. In the first (high) pressure stage the flow medium on itsflow path initially passes through economizers, which use the residualheat to preheat the flow medium, and subsequently different stages ofevaporator and superheater heating surfaces. In the evaporator the flowmedium is evaporated, thereafter any possible residual moisture isseparated in a separation device and the steam left behind is heated upfurther in the superheater. Thereafter the superheated steam flows intothe high-pressure part of the steam turbine, is expanded there andsupplied to the following pressure stages of the steam turbine. It issuperheated again there and supplied to the next pressure section of thesteam turbine.

Because of load fluctuations the heat power transferred to thesuperheater can be heavily influenced. It is therefore frequentlynecessary to regulate the superheating temperature. In new plants thisis mostly achieved by an injection of feed water between the superheatersurfaces for cooling, i.e. an overflow line branches off from the mainflow of the flow medium and leads to injection valves disposedaccordingly there. The injection in such cases is usually regulated viaa characteristic value predetermined for the temperature discrepanciesfrom a predetermined temperature nominal value at the outlet of thesuperheater.

The demands imposed on modern power plants include not only high levelsof efficiency but also a method of operation that is as flexible aspossible. In addition to short start-up times and high load changespeeds, this also includes the option of compensating for frequencydisruptions in the electricity grid. In order to fulfill theserequirements the power plant must be in a position to be able to providepower increases of for example 5% within a few seconds.

This is realized in previous usual combined cycle power plants byincreasing the load on the gas turbines. Under certain circumstanceshowever it can be possible, especially in the upper load range for thedesired power increase not to be able to be provided exclusively by thegas turbines. Therefore in the interim solutions have also been pursuedin which the steam turbine likewise can and is intended to make anadditional contribution to frequency support in the first seconds.

This can occur for example by opening partly throttled turbine valves ofthe steam turbine or of a so-called step valve, through which the steampressure upstream of the steam turbine is reduced. Steam from the steamreservoir of the upstream waste heat steam generator is stored by thisprocess and supplied to the steam turbine. With this measure a powerincrease in the steam part of the combined cycle power plant is achievedwithin a few seconds.

This additional power can be released within a relatively short time sothat the delayed heat power increase by the gas turbines (limited bytheir maximum speed of load change resulting from design and operationalconstraints) can be at least partly compensated for. The entire blockmakes a direct jump in performance through this measure and through asubsequent increase in power of the gas turbine can also maintain orexceed this power level permanently, provided the plant is in the partload range at the point at which the additional power reserves arerequested.

A permanent throttling of the turbine valves to maintain a reservehowever always leads to a loss of efficiency so that, for cost effectiveoperation the degree of throttling should be kept as low as isabsolutely necessary. In addition a few designs of waste heat steamgenerator, thus for example once-through circulation steam generators,under some circumstances have a significantly smaller storage volumethan for example natural circulation steam generators. The difference inthe size of the boiler has an influence in the method described above onthe behavior during changes in power of the steam part of the combinedcycle power plant.

SUMMARY OF THE INVENTION

The object of the invention is thus to specify a method for regulationof a short-term power increase of a steam turbine with an upstreamwaste-heat steam generator of the type given above, in which theefficiency of the steam process is not disproportionately adverselyaffected. At the same time the short-term power increase should be madepossible independently of the design of the waste-heat steam generatorwithout invasive structural modifications to the overall system.

This object is achieved in accordance with the invention, for ashort-term power increase of the steam turbine, by reducing thetemperature nominal value and increasing the characteristic value forthe period of the reduction of the temperature nominal value temporarilydisproportionately to the discrepancy.

The invention is based on the idea that additional injection of feedwater can make a further contribution to a rapid variation in power.Through this additional injection in the area of the superheater thesteam mass flow can namely be temporarily increased. The injection istriggered in such cases by the temperature nominal value being reduced.A jump in the temperature nominal value is linked via a correspondingcharacteristic value to a jump in the regulator discrepancy, whichcauses the regulator to vary the degree of opening of the injectionregulation valve. Thus a power increase of the steam turbine can berealized precisely by such a measure, i.e. a sudden reduction in thetemperature nominal value.

This power increase and thus also the injection mass flow should furtherbe provided as quickly as possible. In such cases however attenuatingproperties of the regulation system can be a hindrance, which preventsdisproportionately fast variations of the injection mass flow which,although desirable in normal load operation, are not desirable for anincrease in power to be provided quickly. Therefore the regulationshould be adapted accordingly for the case of a short-term powerincrease. This is possible in an especially simple manner by theregulation signal for the injection mass flow being amplifiedaccordingly, and by this being done for the period of the desiredshort-term power increase. For this purpose the characteristic value forthe discrepancy between the outlet temperature of the last superheaterheating surface on the flow medium side and a predetermined temperaturenominal value is temporarily increased disproportionately to thediscrepancy for the period of the reduction of the temperature nominalvalue.

In the method described above in a corresponding regulation system anactual-nominal comparison between desired and measured steam temperatureis made via a subtractor element. Depending on the regulation conceptused, this signal can still be modified further by additionalinformation from the process, before it is subsequently switched as aninput signal (regulation discrepancy) to a PI controller for example.Advantageously the temperature can additionally be used as a regulationvariable directly after the injection point of the flow medium, i.e. atthe inlet of the last superheater heating surface. In this type ofso-called dual circuit regulation sudden variations in the injectionmass flow which have occurred as a result of the regulator interventionare damped out. Under these circumstances the regulation optimized forrapid interventions can be stabilized by preventing an overshoot.

For the provision of an immediate reserve by the injection system thisdamping effect of the dual-circuit regulation is however rather ahindrance. Therefore, with dual-circuit regulation in particular, it isespecially advantageous to perform the described amplifying adaptationof the characteristic value. What the artificial regulation-sideincrease of the discrepancy created thereby of the actual temperaturefrom the predetermined nominal value namely achieves is that thesubsequent correction by the temperature at the inlet of the lastsuperheater heating surfaces, i.e. directly after the injection point,ends up comparatively lower in dual-circuit regulation. This means thata greater regulation discrepancy remains, which directly results in agreater regulator response, i.e. a greater increase in the injectionmass flow, which is desired in this case. The fact that thecharacteristic value however is only increased temporarilydisproportionately for the period of the reduction of the temperaturenominal value means that the influence of this overincrease disappearsagain, so that the steam temperature set via the nominal value can alsoactually be achieved. Thus the advantage of dual-circuit regulation, ofavoiding impermissible drops in steam temperature, is still retained.

In an especially simple manner the temporary increase of thecharacteristic value can be advantageously created by the characteristicvalue characteristic for the discrepancy between the temperature and thenominal value being formed from the sum of this discrepancy and a secondcharacteristic value characteristic for the change over time of thetemperature nominal value. In this case, in an especially advantageousembodiment, the second characteristic value is essentially the changeover time of the temperature nominal value multiplied by anamplification factor. In regulation technology terms, this is achievedby the predetermined steam temperature nominal value being used as aninput signal of a first order differentiation element and the output ofthis element being subtracted, after suitable amplification from thedifference between measured and predetermined temperature at the heatersurface outlet. The desired artificial increase of the discrepancy isrealized especially easily in this way and via the additionalfirst-order differentiation element the injection mass flow and thus thepower additionally released via the steam turbine is increasedsignificantly faster.

Because of the differential character, i.e. taking account of the changein the nominal value over time, the influence of such regulation on thesystem as a whole decreases over the course of time (disappearingpulse). This means that the differentiation element has no furtherinfluence on the regulation discrepancy and the current temperature setvia the nominal value is also reached. Even in the event of the nominalvalue of the steam temperature not changing (the normal situation inconventional load operation) such an embodiment has no influence on theremaining regulation structure. Thus in conventional load operation nodifferences occur in the regulation behavior of the steam temperatureregulation between the regulation structure with or without thisadditional differentiation element.

In an advantageous embodiment a parameter of one of the characteristicvalues is determined specifically for the plant. This means that thelevel of the amplification, the parameters of the differentiationelement etc. are to be determined specifically on the basis of the plantconcerned in the individual case. This can be done for example inadvance with the aid of simulation calculations or can happen duringcommissioning of the regulation.

In the waste heat steam generators normally used today there is alsoinjection on the flow medium side downstream from the superheaterheating surfaces into the flow path (end injection). However the use ofthe injection (intermediate injection) described above disposed betweenthe superheater heating surfaces during use for providing a powerreserve produces a higher energy yield, since only here can there be anexplicit utilization of the thermal energy which is stored in theheating surfaces located upstream. However system conditions mean thatit takes some time until the additional intermediate injection makesitself felt in the form of additional power at the steam turbine, sincefirst of all the entire superheater path downstream from theintermediate injection must be loaded up before an increased steam massflow becomes evident as a result of an additional injection at theturbine inlet.

For this reason it is of advantage to also use the final injection andthus also the thermal energy stored in the steam line pipe wall of thefresh steam line to the steam turbine. Because of the fact that thefinal injection is disposed directly at the inlet into this fresh steamline, the reaction namely takes place directly, i.e., when an injectionregulating valve of the final injection is opened, a higher steam massflow is present relatively rapidly at the turbine inlet and thus ensuresa rapid power increase. This however functions only as long as thethermal reservoir of the fresh steam line is not yet fully utilized forthe present application, however this store is expected to be sufficientuntil the additionally obtained power via the intermediate injectioncomes into play. In concrete terms this means that the dead time orreaction time of the intermediate injection in respect of the provisionof the immediate reserve can be effectively compensated for by inclusionof the final injection.

For this purpose the temperature nominal value is also reduced duringthe end injection for a short-term increase in the power of the steamturbine, which is also used here as a regulation variable for the amountof flow medium M injected. This change however is applied in usualsystems with a slight time delay (e.g. in control technology terms by aPTn element). This time delay models the temporal behavior of thesuperheater path between intermediate and final injection, i.e. it isadvantageously characteristic of the throughflow time of the flow mediumM through the superheater heating surfaces and of its thermal behaviorbetween the two injection points. Under these circumstances theintermediate injection regulation valve opens first, since this firstexperiences the change in the temperature nominal value. Because of theinjection amount introduced the temperature before the final injectionreduces with the temporal behavior of the superheater path. Thus in theusually favorable case the final injection is not active, which undercertain circumstances is desirable in conventional operation. If howeverthe final injection is to be used as a result of the said advantages,this must become active directly after the application of the nominalvalue change to the assigned characteristic value. For this purpose thetime delay of the temperature nominal value is advantageouslydeactivated in the definition of characteristic value.

In any event it should be guaranteed for this measure that the finalinjection is optimized in respect of the injection quality, so that afiner spray mist is created. This avoids large water droplets enteringinto the steam turbine and being able to damage the turbine. With anappropriately fine spray mist all water droplets are already evaporatedas soon as they reach the steam turbine.

In an advantageous embodiment a regulation system for a waste heat steamgenerator with a number of economizer, evaporator and superheaterheating surfaces forming a flow path through which a flow medium flowscomprises means for executing the method. In a further advantageousembodiment a waste heat steam generator for a combined cycle power planthas such a regulation system and a combined cycle power plant has such awaste heat steam generator.

The advantages obtained by the invention especially consist of enabling,through the explicit reduction of the steam temperature nominal valueusing the injection regulation method, the thermal energy stored in themetal masses downstream from the injection to be used for a temporarypower increase of the steam turbine. If in such cases the adaptedregulation method described is used, in the event of a sudden reductionof the steam temperature nominal value, significantly faster powerincreases are able to be realized with the aid of the injection system.

In addition the method for providing a temporary power increase of thesteam turbine is independent of other measures, so that also for examplethrottled turbine valves can additionally be opened in order to furtheramplify the power increase of the steam turbine.

The effectiveness of the method remains largely unaffected by theseparallel measures.

In such cases it should be stressed that, for a fixed predeterminedrequirement for additional power, the degree of throttling of theturbine valves can be reduced, should the use of the injection system beapplied for increasing the power. The desired power release can underthese circumstances then be achieved with a lower, in the most favorablecase entirely without, additional throttling. Thus in normal loadoperation in which it must be available for an immediate reserve, theplant can be operated with a comparatively high level of efficiency,which also lowers operational costs.

A further advantage of the method consists of being able to lower theactual steam temperature by the temperature regulation concept. Maximumallowable temperature transients of the steam turbine are not exceededunder these circumstances with a maximum possible power increase.Precisely in respect of the additional use of the final injection, thefresh steam temperature can be adjusted very precisely.

Finally the method is also to be realized without any invasiveconstructional measures, but can merely be undertaken by animplementation of additional components in the regulation system.Greater plant flexibility and benefit without additional costs isachieved by this method.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention will be explained in greaterdetail with reference to a drawing, in which:

FIG. 1 shows a flow-medium-side schematic of the high-pressure part of awaste heat steam generator with data-side connection of the intermediateinjection regulation system for use for an immediate power release, and

FIG. 2 shows a flow-medium-side schematic of the high-pressure part of awaste heat steam generator with data-side connection of the finalinjection regulation system for use for an immediate power release.

DETAILED DESCRIPTION OF THE INVENTION

The same parts are provided in both figures with the same referencecharacters.

FIG. 1 for example shows the high-pressure part of the waste heat steamgenerator 1. The invention can naturally also be used in other pressuresstages for regulation of the intermediate superheating. FIG. 1schematically represents a part of the flow path 2 of the flow medium M.Of the heating surfaces of the economizer, evaporator and superheaterusually disposed in the high-pressure part of the waste heat steamgenerator 1 only the last superheater heating surfaces 4 are shown. Thespatial arrangement of the individual superheater heating surfaces 4 inthe hot gas duct is not shown and can vary. The superheater heatingsurfaces 4 shown can each be representative of a plurality of seriallyconnected heating surfaces which, for reasons of clarity, are not showndifferentiated.

Before it enters the part shown in FIG. 1, the flow medium M is conveyedfrom a feed pump at the corresponding pressure into the high-pressureflow path 2 of the waste heat steam generator 1. During this process theflow medium M initially passes through an economizer, which can comprisea plurality of heating surfaces. The economizer is typically disposed inthe coldest part of the hot gas duct, in order to obtain a use ofresidual heat here to increase the efficiency. Subsequently the flowmedium M passes through the heating surfaces of the evaporator and afirst superheater. In such cases a separation device can be disposedbetween the evaporator and superheater, which removes the residualmoisture from the flow medium M so that only pure steam gets through tothe superheater.

Disposed on the flow medium side downstream from a first superheatingheating surface not shown in the figure is an intermediate injectionvalve 6, a further final injection valve 8 is disposed after the lastsuperheater surface 4. Here cooler and unevaporated flow medium M forregulating the outlet temperature at the outlet 10 of the high-pressurepart of the waste heat generator 1 can be injected. The amount of flowmedium M introduced into the intermediate injection valve 6 is regulatedvia the injection valve 12. The flow medium M in this case is suppliedvia an overflow line 14 branching off beforehand in the flow path 2.Disposed in the flow path 2 for regulating the injection are a number ofmeasurement devices, namely a temperature measurement device 16 upstreamfrom the intermediate injection valve 6, a temperature measurementdevice 18 and a pressure measurement device 20 downstream from theintermediate injection valve 6 and upstream from the superheater heatingsurfaces 4, as well as a temperature measurement device 22 downstreamfrom the superheater heating surfaces 4.

The other parts of FIG. 1 show a regulation system 24 for theintermediate injection. First of all a temperature nominal value is setat a nominal value generator 26. This temperature nominal value isswitched together with the temperature measured at the temperaturemeasurement device 22 downstream from the superheater heating surface 4to a subtractor element 28, where the discrepancy between thetemperature at the outlet of the superheater heating surfaces 4 and thenominal value is thus formed. This discrepancy is corrected in an adderelement 30, wherein the correction models the time delay of atemperature change during passage through the superheater heatingsurfaces 4. To this end the temperature at the inlet of the superheaterheating surfaces 4 is switched from the temperature measurement device18 to a time-delayed PTn element 32. The signal produced is switchedtogether with the value from the temperature measurement device 18 to asubtractor element 34, the output of which is supplied to the adderelement 13. The subtractor element 34 consequently only supplies a valueother than zero for a certain time after a change of the temperature atthe temperature measurement device 18, which corrects the discrepancypresent at the adder element.

The signal present at the adder element 30 is switched together withfurther signals to a minimum element 36, which takes account of furtherparameters: On the one hand the temperature downstream from theintermediate injection must have a certain distance from thepressure-dependent boiling temperature. For this purpose the pressuremeasured at the pressure measurement device 20 is switched into afunctional element 38, which outputs the boiling temperature of the flowmedium M corresponding to this pressure. In an adder element 40 a presetconstant from a generator 42 is added, which can amount for example to30° C. and guarantees a safety gap to the boiling line. The minimumtemperature thus determined is switched together with the actualtemperature determined at the temperature measurement device 18 to asubtractor element 44 and the discrepancy thus determined is given tothe minimum element 36. For reasons of clarity a few switchingconnections are not shown in FIG. 1 but are indicated by appropriateconnection symbols <A>, <B>, <C>.

Furthermore following injection, a certain enthalpy of the flow medium Mmust be guaranteed which must not fall below a certain level foroperational reasons. To this end the signals of the pressure measurementdevice 20 as well as those of the temperature measurement devices 16, 18upstream and downstream from the intermediate injection are switched tothe enthalpy module connected upstream from the minimum element 36. Theenthalpy module 46 for its part calculates an associated temperaturedifference on the basis of these parameters which will be connected asan input signal to the following minimum element 36. The signaldetermined in the minimum element 36 is connected to a regulationelement 48 for controlling the injection regulation valve 12.

To enable the injection system to be used not only for regulation of theoutlet temperature, but also to provide an immediate power reserve, saidsystem comprises corresponding means for executing the method forregulating a short-term power increase of the steam turbine. Initiallyfor this purpose the temperature nominal value is reduced at the nominalvalue generator 26, which results in an increase in the intermediateinjection amount. However so that this leads directly to a powerincrease the, a rapid regulator response of the PI regulator element 48should be guaranteed. The discrepancy caused between the actualtemperature and the temperature nominal value will however beameliorated by the PTn element 32 shortly after the change.

In order to prevent this in the event of a desired rapid power increase,the signal of the nominal value generator 26 for the temperature nominalvalue is to be switched to a first-order differentiation element (DT1).For this a PT1 element 50 has the signal of the nominal value generator26 applied to it on the input side and is switched together with theoriginal signal of the nominal value generator 26 to a subtractorelement 52 on the output side, the output of which is connected to amultiplier element 54 which amplifies the signal by a factor, e.g.5,from a generator 56. This signal is in its turn added by a subtractorelement 58 into the signal to the adder element 30. In the event of achange in the nominal value, the circuit outputs a signal differing fromone via the PT1 element 50, which is amplified by the multiplier element54 and artificially disproportionately amplifies the valuecharacteristic for the discrepancy. The signal via the loop with the PTnelement 32 is then relatively smaller and a faster regulator response ofthe PI regulator element 48 is forced. Thus a steam amount increase israpidly achieved and the power of the downstream steam turbine isincreased.

FIG. 2 now shows the parts of the regulation system 24 relevant to thefinal injection. There is a further temperature measurement device 60here in the flow path 2 downstream from the final injection valve 8Likewise the temperature nominal value of the nominal value generator 26is used here as a regulation variable. Its signal is sent to a PTnelement 62 which, like the PTn element 32, models the time delay throughthe superheater heating surfaces 4. Its output signal is sent togetherwith the signal of the nominal value generator 26 to a maximum element64, of which the output signal together with the signal from thetemperature measurement device 60 is sent into a subtractor element 66.The discrepancy determined there is sent to a PI regulation element 68which regulates the injection regulation valve 70 of the finalinjection.

In the event of a change in the temperature nominal value via thenominal value generator 26, the PTn element, in combination with themaximum element 64, here delays the regulator response of the PIregulator element 68. In order to prevent this for the case of a finalinjection desired quickly, the time delay, i.e. the PTn element 62, isdeactivated for a time in such a case. This speeds up the regulatorresponse accordingly and a fast power release is possible.

A waste heat steam generator 1 regulated in this way is now used in acombined cycle power plant. Here the hot waste gases of one or more gasturbines are routed on the flue gas side through the waste heat steamgenerator 1, which thus provides steam for a steam turbine. The steamturbine in this case comprises a number of pressure stages, i.e. thesteam heated up by the high-pressure part of the waste heat steamgenerator 1 and expanded in the first stage (high-pressure stage) of thesteam turbine is routed into a medium-pressure stage of the waste heatsteam generator 1 and is superheated there once again, but to a lowerpressure level however. As already mentioned, the exemplary embodimentshows the high-pressure part of the waste heat steam generator 1 toillustrate the invention by way of example, but this can also be used inother pressure stages.

A combined cycle power plant equipped with such a waste heat steamgenerator is able not only to provide a short-term power increase of thegas turbine which is restricted by the allowed maximum load changespeed, but also to rapidly provide a power increase via an immediatepower release of the steam turbine, which serves to support thefrequency of the electricity grid.

The fact that this power reserve is achieved by a double use of theinjection valves as well as the usual temperature regulation alsoenables a permanent throttling of the steam turbine valves in order toprovide a reserve to be reduced or dispensed with entirely, whereby anespecially high level of efficiency during normal operation is achieved.

1.-10. (canceled)
 11. A method for regulating a short-term powerincrease of a steam turbine with an upstream waste heat steam generator,comprising: forming a flow path through which a flow medium flows by aplurality of economizer, evaporator and superheater heating surfaces ofthe waste heat steam generator; branching off the flow medium in apressure stage from the flow path; injecting the flow medium into theflow path between two superheater heating surfaces of the pressure stageon a flow medium side; regulating amount of the injected flow medium bya first characteristic value, wherein the first characteristic value isdetermined by a discrepancy between an outlet temperature of asuperheater heating surface on the flow medium side of the pressurestage from a predetermined temperature nominal value; reducing thetemperature nominal value for the short-term power increase of the steamturbine; and temporarily increasing the first characteristic valuedisproportionately to the discrepancy for a period of the reduction ofthe temperature nominal value.
 12. The method as claimed in claim 11,wherein the amount of the injected flow medium is further regulated by atemperature directly downstream from injection point of the flow medium.13. The method as claimed in claim 11, wherein the first characteristicvalue comprises a sum of the discrepancy and a second characteristicvalue, wherein the second characteristic value is a change over time ofthe temperature nominal value.
 14. The method as claimed in claim 13,wherein the second characteristic value is the change over time of thetemperature nominal value multiplied by an amplification factor.
 15. Themethod as claimed in claim 13, wherein a parameter of the first or thesecond characteristic values is determined for a power plant.
 16. Themethod as claimed in claim 11, wherein the flow medium is injected intothe flow path on the flow medium side downstream from the superheaterheating surfaces, and wherein a time delay of the temperature nominalvalue is deactivated in determining the first characteristic value. 17.The method as claimed in claim 16, wherein the time delay ischaracteristic for a throughflow time of the flow medium through thesuperheater heating surfaces between two injection points and/or thermalbehavior of the superheater heating surfaces.
 18. A waste heat steamgenerator for a combined cycle power plant, comprising: a plurality ofeconomizer, evaporator and superheater heating surfaces forming a flowpath through which a flow medium flows; and a regulation system adaptedto perform the method as claimed in claim
 11. 19. A combined cycle powerplant, comprising: a waste heat steam generator as claimed in claim 18.